Sustained strong winds will accelerate slab formation and keep loading lee terrain even when the snow stops falling. Storm slabs should be primed for human triggering on Friday.
Weather Forecast
Friday: Cloudy with scattered flurries bringing a trace to 5 cm of new snow. Moderate southwest winds. Freezing level near valley bottom with alpine high temperatures around -6.Saturday: Mainly cloudy. Moderate southwest winds. Freezing level near valley bottom with alpine high temperatures around -7Sunday: Cloudy with scattered flurries and 2-5 of new snow. Strong southwest winds. Freezing level near valley bottom with alpine temperatures around -7.
Avalanche Summary
Thursday's reports include only one Size 2 natural storm slab release. That said, heavy snowfall has been obscuring visibility into most terrain.Very significant avalanche activity occurred in the region last week. Large avalanches ran on multiple persistent weak layers, even in fairly dense trees and low angle slopes. Activity has gradually tapered off, but the potential for human triggering remains. On Tuesday, explosive control in the Lizard range produced a few large persistent slab avalanches on deep weak layers (size 2-2.5). Recent natural activity was also reported in the Sparwood area.Looking forward, new snow from Thursday's storm has introduced both an increased load to the snowpack as well as the possibility for storm slab releases to act as a triggering mechanism for deeper weak layers. Expect ongoing strong winds to continue loading lee aspects even as snowfall tapers off. These winds will also be promoting slab formation and reactivity.
Snowpack Summary
About 30 cm of new snow over Wednesday night and Thursday has buried a new layer of feathery surface hoar on sheltered aspects (especially prominent from 1400-1900 m) as well as sun crust on solar aspects. The new snow will need time to form a reliable bond to this previous surface. The recent warm temperatures that created the crust also worked to settle the upper snowpack. Shifting winds during the warm period transported loose snow into wind slabs in open terrain that are now buried by new snow. The winds also built up cornices along ridges. A number of buried weak layers have been very concerning over the past week, and appear to be gaining strength very slowly. The early January surface hoar layer is around 60 cm below the surface and the unstable weak layer from mid-December (predominantly surface hoar and/or a sun crust) is around 100 cm below the surface at treeline and below treeline elevations. A rain crust with sugary facets that developed late-November is near the bottom of the snowpack. All of these layers have been reactive over the past week.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.